---
title: Optics
description: This doc describes Optics-ts extension.
nav: 4.09
keywords: optics
---

### Install

You have to install `optics-ts` and `jotai-optics` to use this feature.

```
npm i optics-ts jotai-optics
```

## focusAtom

`focusAtom` creates a new atom, based on the focus that you pass to it. This creates a derived atom that will focus on the specified part of the atom,
and when the derived atom is updated, the derivee is notified of the update, and the equivalent update is done on the derivee.

See this:

```js
const baseAtom = atom({ a: 5 }) // PrimitiveAtom<{a: number}>
const derivedAtom = focusAtom(baseAtom, (optic) => optic.prop('a')) // PrimitiveAtom<number>
```

So basically, we started with a `PrimitiveAtom<{a: number}>`, which has a getter and a setter, and then used `focusAtom` to zoom in on the `a`-property of
the `baseAtom`, and got a `PrimitiveAtom<number>`. What is noteworthy here is that this `derivedAtom` is not only a getter, it is also a setter. If `derivedAtom` is updated, then equivalent update is done on the `baseAtom`.

The example below is simple, but it's a starting point. `focusAtom` supports many kinds of optics, including `Lens`, `Prism`, `Isomorphism`.

To see more advanced optics, please see the example at: https://github.com/akheron/optics-ts

### Example

```jsx
import { atom } from 'jotai'
import { focusAtom } from 'jotai-optics'

const objectAtom = atom({ a: 5, b: 10 })
const aAtom = focusAtom(objectAtom, (optic) => optic.prop('a'))
const bAtom = focusAtom(objectAtom, (optic) => optic.prop('b'))

const Controls = () => {
  const [a, setA] = useAtom(aAtom)
  const [b, setB] = useAtom(bAtom)
  return (
    <div>
      <span>Value of a: {a}</span>
      <span>Value of b: {b}</span>
      <button onClick={() => setA((oldA) => oldA + 1)}>Increment a</button>
      <button onClick={() => setB((oldB) => oldB + 1)}>Increment b</button>
    </div>
  )
}
```

#### Codesandbox

<CodeSandbox id="nsy4u8" />
